Scientists have found evidence that a surprising amount of pack ice in the Arctic Ocean melted last summer. But experts don't know whether to blame the "greenhouse effect" or the Arctic's normal climate fluctuations for the melting, as reported in the 15 May Geophysical Research Letters.
Climate models predict that temperatures in Earth's polar regions will rise faster than the rest of the planet in response to global warming. Researchers suspect this trend could lead to a slow but accelerating loss of ice, especially over the ocean. When patches of ice melt, the water underneath absorbs more solar heat, which in turn triggers more melting nearby. Indeed, satellite images show that the total area covered by Arctic ice is about 6% smaller than it was 20 years ago. But fewer records exist on the ice's thickness, a critical measure of its total volume.
Scientists aboard an icebreaker that churned 500 kilometers north of Alaska last September were startled by the lack of hardy ice. Instead of the usual thickness of 2 to 3 meters in the autumn, they found few floes thicker than 1.5 meters, says oceanographer Miles McPhee of the McPhee Research Company in Naches, Washington. After setting up a research station on the pack ice last fall, McPhee and his colleagues probed the ocean beneath it. The water was far less salty, and about half a degree Celsius warmer, than it was in a similar study in 1975. The amount of fresh water, and the fact that it has not diffused far below the base of the ice, both suggest that a great deal of melting may have occurred in just one season, rather than over many years, McPhee says. "That's the alarming thing about it," he says, because the warm water could mean less ice this winter--and more melting next summer.
Submarine sonar measurements also revealed that Arctic ice was about half as thick last autumn as it was in 1993, notes oceanographer Richard Moritz of the University of Washington, Seattle, lead scientist for the Surface Heat Budget of the Arctic (SHEBA ) research station. However, he cautions, the data on ice thickness are too sparse to define a climate trend. Further, a pulse of atmospheric warmth from El Niño complicates the analysis. "We may see the ice make a comeback in a year when heat transport into the Arctic is not as high," Moritz says.